Environmental considerations, particularly the burden of noise pollution to persons in the vicinity of airports when aircraft take off and land, are critical for the right to operate for some airports located in heavily urban areas, where the balance of convenience to the users of the airport is yielding to the objections of the persons exposed to the noise.
In aircraft design engineering, therefore, the search for solutions providing lower noise levels is a constant challenge.
The principal sources of aircraft noise affecting airport environments are generally:                aerodynamic noise in the approach phase, particularly aerodynamic noise generated by high lift enhancing devices and landing gear; and        noise from propulsion units, particularly during takeoff phases, and particularly in the case of high speed aircraft.        
Numerous attempts have been made to devise means for reduction of aerodynamic noise, and to design engines and engine housings (nacelles and other cowling structures) which are intrinsically less noisy.
Attempts to reduce aerodynamic noise by simplifying high lift systems: trailing edge flaps and leading edge slats, are limited by the need for high lift coefficients.
Modern jet aircraft have engines which are, inter alia, less noisy. These engines are of the “turbofan” type, and employ very high dilution, generally greater than 5, which reduces the mean jet velocity of the engine and thereby the noise generated.
Such engines also have low noise nacelles and exhaust nozzles. Eg a forwardly elongated nacelle has the effect of “masking” the noise radiated and propagated from air intake, at least in the direction of the ground. Also (or alternatively), such engines tend to mix core flow and fan air flow, eg with the use of suitable nozzles, which achieves noise reduction in that the core flow is the chief generator of noise.
However, the reduction in noise level which is obtained is often at the cost of increased engine complexity, increased weight, and decreased propulsive efficiency of the propulsive system, which can limit the feasibility of some solutions; moreover, the known solutions are still insufficient to meet future low noise requirements.
Another solution for reducing the noise impact on the ground of aircraft in flight involves locating the engines such that noise radiated from the air intakes and exhaust nozzles are stopped by certain surfaces defined by structures of the aircraft.
In this connection, it has been proposed to mount the engines on the upper surface of the wing structure, achieved with the use of conventional engine nacelles mounted with the aid of vertical pylons which extend between the respective engines and the top surface of the wing structure.
With such an arrangement, the wing surfaces forwardly of and also laterally of the engine air intakes create a barrier which limits propagation of noise toward the bottom of the aircraft and thus toward the ground, when the aircraft is in flight. However, such positioning of the engines where conventional engine nacelles are employed is not sufficient to significantly reduce emitted and propagated noise, particularly toward the rear.
Solutions of the type described above have only rarely been implemented in practice on aircraft.